The invention relates to a process for the preparation of aqueous dispersions of latex particles having a heterogeneous morphology. The invention also relates to the latex particles preparable with the process, the aqueous dispersions and redispersible powders, as well as the use thereof.
Emulsion polymerization is a known procedure for the production of latex polymers having a clearly defined structure. In this connection particular interest is placed on a process leading to the production of so-called heterogeneous latex morphologies. As a result of their molecular structure, such latex particles provide special characteristics and a wide range of uses. In this heterogeneous structure of the latex particles, a distinction is made between a relatively hydrophobic and a relatively hydrophilic area. For thermodynamic equilibrium reasons, under normal conditions there is a hydrophilic shell (outer phase) and a hydrophobic core (inner phase), because these can be more easily produced in general by emulsion polymerization. However, it is much more difficult to produce systems with a so-called inverse core-shell structure, in which the core is hydrophilic and the shell hydrophobic. Increasing importance is being attached thereto as a result of the possible encapsulation of functionalized, hydrophilic polymers.
Such a structure of polymer particles with inverse core-shell structure and their production by emulsion polymerization are described by EP 426 391 A2. According to the process described therein 2 to 60 parts by weight in the form of the core polymer (A) are produced by the emulsion polymerization of 10 to 80 parts by weight of an acrylate ester with an alkyl group with 1 to 3 carbon atoms having 90 to 20 parts by weight of a further monomer. The shell polymer (B) is subsequently produced, e.g. by emulsion polymerization from 98 to 40 parts by weight of at least one vinyl monomer, after which the core polymer is hydrolyzed in alkaline manner. Preferably the shell polymer has a glass transition temperature of at least 50xc2x0 C. Mention is also made of surfactants, which can optionally be used in a quantity of 0.1 to 10 wt. %. The particles are used as a pigment or filler in paints and paper coatings. An inadequate storage stability is a disadvantage of the dispersions described. In addition, the dispersed core-shell particles do not have a satisfactory resistance to water, so that after a certain time it is no longer possible to use them. The particle sizes which can be produced with the process of EP 426 391 A1 are also very ununiform and spread over a wide range, so that the uniform particle sizes necessary in certain applications are not available.
An encapsulation of hydrophilic polymers and the production thereof are also described in EP 696 602 A1. In the latter, the hydrophilic core is formed from 5 to 100 wt. % hydrophilic monomer and 0 to 95 wt. % nonionic monomer and the hydrophobic shell of 90 to 99.99 wt. % nonionic monomer and 0.1 to 10 wt. % acid-functionalized monomer. The acid-functionalized monomer is a carboxylic acid monomer, preference being given to (meth)acrylic acid. It is also possible to use non-polymerizable carboxylic acids, such as C6-C12 aliphatic or aromatic monocarboxylic or dicarboxylic acids. Styrene is mentioned as the preferred nonionic monomer for the shell. The particle size is preferably between 50 and 2000 nm. Additionally the core can contain less than 20, preferably 0.1 to 3 wt. % of multiply unsaturated monomers or 0.1 to 60 wt. % butadiene.
In the preparation process of EP 696 602 A1, the time sequence during the addition of the acid monomer in preparing the shell is of vital importance. If the core particle size is below 130 nm, the acid monomer must be added during the first 50% of the addition of the total shell monomer, preferably during the first 25% and in particularly preferred manner during the first 10%. If the core particle size exceeds 130 nm, the acid monomer must be added during the 100% addition of the total, added shell monomer, preferably during the first 50%, in particularly preferred manner during the first 25% and more especially during the first 10%. The core polymer can also be prepared by means of a seed, the average particle size being about 30 to 200 nm. Optionally both anionic and nonionic emulsifiers can be used, it being possible to use 0 to 0.75 wt. % emulsifier, based on the total core polymer, for the preparation of the core. It is unnecessary to add additional emulsifier during the preparation of the shell, but mention is made of tile addition of 0.05 to 2 wt. %, based on the total shell polymer weight. By adding bases the acid functions of the core can be neutralized and then the core swells, the polymers diffuse out of the core and particles with voids are formed. The latex particles prepared with the voids produced are more particularly used in aqueous coating compositions, such as water-based paints and paper coatings and give the latter brightness and body. A disadvantage of this process is the timing which must be carefully respected during the addition of the acid polymer for polymerizing the shell polymer, as well as the dependence of the alternative procedure on the core polymer particle size. The polymerization process is also very complex and technically complicated as a result of the described alternatives. The emulsifiers are only optionally used, mention only being made of anionic and nonionic emulsifiers.
The problem of the invention is to so further develop the aqueous dispersions or latex particles prepared according to the aforementioned processes, that whilst maintaining advantageous characteristics or in individual cases providing improved characteristics, the process control and the selection of the starting materials can be made more flexible, whilst simultaneously providing a simplified production procedure. In addition, the preparable latex particles, both in dispersed form and in powder form, must have a good storage stability and water resistance. In addition, these dispersions or the dispersed latex particles must be improved in such a way that during their final applications, such as e.g. in plastics-containing, cement-bound systems, as a result of desirable consecutive reactions, they lead to advantageous use products with improved processability and characteristics.
According to the invention, the above problem is solved by a process for the preparation of aqueous dispersions of latex particles having a heterogeneous morphology by a semicontinuous emulsion polymerization, comprising the emulsion polymerizing of ethylenically unsaturated (co)monomers, accompanied by the addition of cationic and/or anionic and/or nonionic emulsifiers and/or protective colloids as stabilizers, which are directly used as such or synthesized in situ, the semicontinuous emulsion polymerization being performed in the presence of the stabilizer or stabilizers with a monomer mixture, which
a) contains at least one nonionic, ethylenically unsaturated monomer with a glass transition temperature Tg above about 30xc2x0 C. in a quantity of about 10 to 70 wt. %, based on the total weight of ethylenically unsaturated (co)monomers and
b) at least one hydrophilic, ethylenically unsaturated monomer in a quantity of about 5 to 30 wt. %, based on the total weight of ethylenically unsaturated (co)monomers.